'stats:same'=>'do {my $v1=#HVAL#; my $v2=#_#; if (defined $v1) { #HVAL#=#nan# if $v1!=$v2; } else { #HVAL#= $v2 } }', #hval=nan if $v1!=$v2 works both if nan==nan or nan!=nan : set hval to nan if either one of them is nan or if they are not equal. That way no need to use #v_is_nan#, which would be complicated as it uses $v